Short answer
A 4.8 x 3.6 m deck is 17.3 m². With 90 x 22 mm treated pine boards, a 5 mm gap and 10% waste, it needs about 200.1 lineal metres of boards, or 56 boards of 3.6 m. That deck also needs 12 joists at 450 mm spacing and roughly 958 screws. Materials come to about $1,656 to $2,172, against roughly $3,600 to $6,900 if a carpenter builds it. The calculator below works for any size, board and framing.
Use this free decking calculator to work out how many boards, joists, bearers, footings and screws a deck needs, plus an indicative materials cost and what a carpenter might charge. Choose your board, adjust the gap and spacing, and the quantities update as you go.
Decking calculator
Board quantities are calculated from your sizes. Joist, bearer and footing sizes and spans depend on the AS 1684 span tables and, for some decks, an engineer, so use the framing figures as a starting point only. Prices are indicative Bunnings prices. Ranges reviewed October 2026.
How to calculate decking boards
- Deck area: length x width in metres.
- Row width: board cover width plus the gap between boards. A 90 mm board with a 5 mm gap covers 95 mm.
- Lineal metres of board: area ÷ row width in metres, then add 10% waste for straight lay or 15% for a diagonal lay.
- Number of boards: lineal metres ÷ the board length you buy, rounded up. Rows longer than the board need butt joins on a joist, staggered between rows.
- Joists: deck length ÷ joist spacing, plus one. Joists run across the boards.
- Screws: two per board at every joist, so rows x joists x 2, plus a little spare.
| 4.8 x 3.6 m deck (17.3 m²), 10% waste | Boards to buy | Joists | Screws | Materials, indicative |
|---|---|---|---|---|
| Treated pine 90 x 22 mm, 3.6 m boards | 56 | 12 | 958 | $1,656 to $2,172 |
| Merbau 140 x 19 mm, 5.4 m boards | 25 | 12 | 630 | $2,166 to $2,598 |
| Composite 137 x 23 mm, 5.4 m boards | 25 | 13 | 710 | $2,825 to $5,363 |
Joist spacing in the table uses 450 mm for timber and 400 mm for composite, and the framing is 140 x 45 mm H3 pine with bearers 1.5 m apart. Materials include boards, framing and screws, but not footings, concrete, balustrades, stairs, oil or stain.
Decking board sizes and prices
These are common board sizes and Bunnings prices seen in October 2026. Prices vary by store and length, and some boards are cheaper as finger-jointed or random lengths.
| Board | Typical lengths | Price per lineal metre |
|---|---|---|
| Treated pine 90 x 22 mm (H3) | 2.4 to 3.6 m | about $4 to $5.60 |
| Merbau 90 x 19 mm | 2.4 m to 5.7 m | about $5 to $7 |
| Merbau 140 x 19 mm | 2.4 m to 5.7 m | about $10 to $12 |
| Spotted gum 86 x 19 mm | to about 5.7 m | about $10.50 to $11 |
| Spotted gum 135 x 19 mm | to about 5.7 m | about $18 to $19 |
| Blackbutt 86 x 19 mm | to about 5.7 m | about $10 to $10.50 |
| Composite 137 x 23 mm (Ekodeck, Modwood) | 5.4 m | about $14.50 to $32 |
| H3 pine joists and bearers 90 x 45 / 140 x 45 / 190 x 45 mm | various | about $7.50 to $8.50 / $11.50 to $13 / $16 to $17.50 |
Stainless 304 decking screws (10G x 50 mm) cost about $0.10 to $0.20 each depending on the pack size. We did not find a Bunnings price for 140 x 22 mm treated pine decking, so it is not in the calculator.
Board gaps and joist spacing
These figures are from the WoodSolutions Technical Design Guide 21 (domestic timber deck design), which sets out AS 1684 spacing, and from composite manufacturers.
| Board | Maximum joist spacing | Gap between boards |
|---|---|---|
| 19 mm hardwood | 500 mm | 3 to 4 mm (boards up to about 90 mm), 4 to 5 mm (up to about 140 mm) |
| 19 mm treated pine | 400 mm | about 5 mm |
| 22 mm treated pine | 450 mm | about 5 mm |
| Any board laid on the diagonal | about 350 mm | as above |
| Composite (137 to 138 mm) | 400 to 450 mm, by maker | about 4 to 5.5 mm, by maker |
The calculator starts at 450 mm for 19 mm hardwood, which is more cautious than the 500 mm maximum. Timber boards of 25 to 32 mm can span further, but we did not find a standard figure, so check the supplier’s span table.
Framing, footings and durability
- Joists and bearers: sizes and spans come from the AS 1684 span tables, and an engineer is needed for tiled decks, spas, roofs, elevated decks and cantilevers. The calculator asks for your bearer and footing spacing instead of assuming one.
- Hazard class: H3 treated pine is the minimum above ground. Framing within about 150 mm of the ground needs a higher class (H4 in the WoodSolutions guide, and H5 in Timber Queensland guidance), and posts in the ground need H5.
- Footings: use the concrete calculator to work out the concrete for post holes, and keep at least 40 mm of airspace between the frame and the house for ventilation.
- Fixings: two screws per board at every joist, with 10G x 50 mm screws common for 19 and 22 mm boards. Use hot-dip galvanised or stainless, and 316 stainless near the coast or a pool.
Rules, approvals and bushfire areas
- NSW: a deck up to 25 m² with a floor no more than 1 m above the ground can be exempt development, with conditions including a 900 mm boundary setback (more in rural zones) and non-combustible materials within 5 m of a house on bushfire-prone land. See the NSW Planning Portal rules for balconies, decks and patios.
- Victoria and Queensland: generally a freestanding deck of 10 m² or less and 800 mm or less above the ground is exempt in Victoria, and Queensland approval is generally needed over 10 m² or 1 m high. Councils add overlays and setbacks, so check with yours.
- Barriers: the National Construction Code requires a barrier where the fall is 1 m or more, at least 1 m high with openings under 125 mm.
- Bushfire areas: treated pine is not rated as bushfire-resisting, while spotted gum, blackbutt and merbau are listed up to BAL-29. Higher ratings need other materials, so check AS 3959 and your council.
The council approval checker gives a quick answer for NSW, Victoria and Queensland.
What does a deck cost installed?
If you hire a carpenter, the price includes framing, footings, boards and labour. These are indicative installed costs per square metre from published 2026 cost guides. Nearly all of them come from contractors and lead-generation sites, which tend to run high, so get written quotes.
| Deck | Indicative installed cost per m² |
|---|---|
| Treated pine | $208 to $398 |
| Merbau or other hardwood | $290 to $550 |
| Composite | about $300 to $580 (up to about $735 for premium boards) |
See the carpenter rates page for hourly rates and the extension cost calculator for larger projects.
Common mistakes
- Ordering the exact area. Cutting, offcuts and damaged boards need 10% extra for a straight lay and more for diagonal or cut-up decks.
- Joists too far apart. Spans that are too wide make boards bounce, split and cup. Diagonal boards need joists closer together.
- No gap between boards. Timber swells and shrinks, and gaps let water drain and air circulate.
- Wrong timber hazard class. Pine framing near the ground needs a higher hazard class than boards above it.
- Using the wrong screws. Zinc-plated screws rust in treated timber. Use stainless or hot-dip galvanised.
- Forgetting the barrier. A fall of 1 m or more needs a compliant balustrade.
- Skipping approval checks. Size, height and boundary distance decide whether you need approval.
Frequently asked questions
How many decking boards do I need for a 4.8 x 3.6 m deck?
With 90 x 22 mm treated pine boards, a 5 mm gap and 10% waste, about 200.1 lineal metres of board, which is 56 lengths of 3.6 m. A 137 x 23 mm composite board at 5.4 m needs about 25 boards for the same deck, because each board covers more of the surface.
How far apart should deck joists be?
It depends on the board. AS 1684, as set out in the WoodSolutions deck guide, allows 500 mm for 19 mm hardwood boards, 450 mm for 22 mm treated pine and 400 mm for 19 mm treated pine, and joists need to be at about 350 mm or closer if boards are laid on the diagonal. Composite makers give their own figures, typically 400 to 450 mm.
What gap should I leave between deck boards?
For seasoned timber, the WoodSolutions deck guide gives 3 to 4 mm for boards up to about 90 mm wide and 4 to 5 mm for boards up to about 140 mm. Softwood is usually laid at about 5 mm. Composite makers specify about 4 to 5.5 mm, so follow the installation guide for your product.
How many screws do I need for a deck?
Two screws in every board at every joist. The 4.8 x 3.6 m treated pine example has 38 rows of boards across 12 joists, which is about 958 screws including 5% spare. Use 10G x 50 mm screws for 19 and 22 mm boards, stainless or hot-dip galvanised rather than zinc-plated, and 316 stainless near the coast.
Do I need approval to build a deck?
Often not for a small, low deck, but the rules differ by state. In NSW a deck up to 25 m² and no more than 1 m above the ground can be exempt development if it meets boundary and other conditions. In Victoria a freestanding deck of 10 m² or less and 800 mm or less above the ground is generally exempt, and a deck attached to the house needs a permit. In Queensland building approval is generally needed over 10 m² or 1 m high. Use the council approval checker, and confirm with your council.
Does a deck need a balustrade?
A barrier is required where the fall from the deck is 1 m or more under the National Construction Code 2022. It must be 1 m high with openings under 125 mm. Lower decks do not need one under the code, but you may still choose to fit one for safety.
Which decking material should I choose?
Treated pine is the cheapest and suits above-ground decks, but it is not rated as bushfire-resisting. Hardwoods such as spotted gum, blackbutt and merbau cost more, and spotted gum, blackbutt and merbau are listed as bushfire-resisting up to BAL-29 in the WoodSolutions guide. Composites cost the most per metre but need less maintenance and have their own spacing and fixing rules.
Related tools
- Concrete calculator - post holes and footings
- Carpenter rates - deck and pergola job prices
- Council approval checker - is your deck exempt?
- Renovation quote comparator - paste deck quotes for red-flag analysis
Related reading: Pergola Cost Australia (2026) (what a pergola costs by size and material).
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